1. Field of the Invention
This invention relates to an auxiliary illumination device to be mounted on a photographic camera for illuminating an object to be photographed and, more particularly, to an illumination device adapted for distance measurement by a camera automatic focusing device when the brightness of the object is low.
2. Description of the Prior Art
Heretofore, there have been proposed various kinds of cameras of the type using a device for automatic focusing (hereinafter called AF). However, the distance measuring power of such AF devices is generally limited. It is particularly difficult to have them perform an AF action on an object of low brightness. To solve this problem, there have also been proposed various improvements. None of these proposed improvements, however, has been satisfactory. In one of these prior art methods, the camera is equipped with an auxiliary illumination device and an object, at the distance which is to be measured, is illuminated, if necessary, from the camera. In other words, a light beam is emitted from the camera in the direction of the optical axis thereof. FIGS. 1, 2 and 3 of the accompanying drawings show some examples of the prior art methods for equipping a camera with such an auxiliary illumination devices for AF. In FIG. 1, an auxiliary illumination device is disposed in front of a pentaprism. This arrangement, however, makes the size of the pentaprism larger and it is not suitable for a camera which must be of a compact size. Besides, in the case of a single lens reflex camera, there is the possibility that an auxiliary light thus obtained might be eclipsed or shade by a lens barrel, as shown in FIG. 2, when a large aperture lens such as a telephoto lens or the like is mounted on the camera. A method of arranging the auxiliary illumination device in the neighborhood of a camera lens, as shown in FIG. 3, not only results in an increased camera size but also incurs the fear of having the auxiliary light eclipsed when a telephoto lens of a large aperture is mounted on the camera. Furthermore, the method of FIG. 3 has another disadvantage in respect to operability as it imposes some restriction on the manner in which the camera can be hand held during photography. As will be understood from the above, is is inevitable that the camera size be greater with an auxiliary illumination device provided on the camera body, in accordance with the methods of the prior art.
The AF camera using an auxiliary illumination device of the above stated type further presents the following problem. While the AF mechanism of the camera is in operation, if either the brightness of the object to be photographed is measured or an aperture or shutter speed is computed concurrently, illumination with auxiliary light would give an erroneous brightness measurement and thus would result in an inadequate exposure. To solve this problem, there is a method of using an auxiliary light that is outside the photosensitive spectrum range for a light sensitive element, such as near infrared rays. However, it is impossible to completely remove infrared rays with any photometric infrared cutting filters that are presently available. Furthermore, when this auxiliary illumination light is emitted during photography, the film would be affected by undesirable illumination if film that is sensitive to the auxiliary light is used.